A battery charger is a type of system for use in charging a rechargeable battery so that the battery may be recharged or its charge maintained. Some battery charging systems are capable of providing power from a single power supply to both a load and to a battery. Examples of loads include modems, computers, telephones, and anything else capable of drawing electrical power. In such battery charging systems, the battery may be used to provide standby power to the load in the event of loss of power from the power supply.
Many types of power supplies are current limited, such as, for example, AC to DC power converters. For example, many AC to DC power converters include foldback current limiting protection circuitry. This protection circuitry limits the amount of current at increasing overloads.
In some situations, the power supply may be unable to provide sufficient power to both the load and to charge the battery. For example, the load may be temporarily increased so that a large amount of current is being drawn from the power supply. If the battery charger attempts to use the maximum charging current to power the increased load and the battery, it may draw too much current and cause the power supply to have a reduced output voltage. This may result in the supported load falling out of regulation.
As an example, FIGS. 1-4 illustrate exemplary curves for four different types of power sources. FIG. 1 illustrates a voltage versus current curve for a power supply with gradual current limit. As illustrated, the voltage gradually falls from 100% to about 90% of maximum as the current approaches 100% of the rating of the power supply. Beyond 100%, the voltage drops dramatically. FIG. 2 illustrates an exemplary voltage versus current curve for a power supply with a hard current limit. As illustrated, this voltage stays at approximately 100% as the current approaches 100% of the rating of the power supply, but drops off to 0 beyond this point. FIG. 3 illustrates an exemplary voltage versus current curve for an unregulated power supply while FIG. 4 shows a curve for a fold-back current limit. All of the power supplies illustrated in FIGS. 1-4 share the common feature that as the current is increased, the voltage drops as the power supply crosses 100% of the rating of the power supply.
In prior systems, a switch was used to provide power to the battery and the load. This switch had two levels, on or off. That is, if the switch was on, the battery was charged at a designed rate, and if it was off, the battery was not charged. In this system, when the power drawn by the load exceeded a threshold, the switch was turned off, thus turning off all charging power to the battery. Thus, if this high power condition persisted for a sufficient period of time, it could result in the battery becoming discharged and the system not having available standby power. This was true regardless of the amount of power drawn by the load over the threshold. Thus, the situation could arise where the switch was completely turned off even though the power drawn by the load only exceeded the threshold by a small amount.